The strong inorganic oxy-acids are very hygroscopic and labile in pure form whether they are crystalline--such as the phosphoric acids--or liquid--such as sulfuric acid and nitric acid--. So these acids are stored or applied almost only in their aqueous solutions.
In practice there is a need for such oxy-acids which are solid, not hygroscopic and storable as such or in mixtures with materials compatible with the acids from which the inorganic oxy-acids can be released by the effect of water or by a relatively strong heating.
The inclusion complexes of inorganic oxy-acids and cyclodextrins according to our invention comply with the requirements of stability and storage and at the same time the effect of the included oxy-acid comes into full display either in the presence of water used for the dissociation of the inclusion complex or in the absence of water by reaching the temperature necessary for decomposing the cyclodextrin structure.
The Hungarian patent No. 172 936 describes the preparation of inclusion complexes of hydrogen halides and cyclodextrins. The essence of this process is that an aqueous solution of a hydrogen halide is mixed with an aqueous solutions of a cyclodextrin while cooling and the inclusion complex of the hydrogen halide and the cyclodextrin so obtained is crystallized by cooling. In this way cyclodextrin complexes with other acids--such as oxy-acids--either cannot be produced or can be obtained only in a very low yield.
Because of attack by strong acids the glycoside bonds of cyclodextrins will be hydrolized (Acta Chim. Hung., 94, 375 (1977)). So in aqueous solutions, depending on the concentration of the acids, the inorganic oxy-acids either cause the hydrolysis of cyclodextrins (in relatively dilute aqueous solutions) or decompose them if the system contains a small amount of water. In any event the inorganic oxy-acids increase the water solubility of cyclodextrins so complexes do not separate from these solutions.
OBJECT OF THE INVENTION
The object of our invention is to provide crystalline, stable inclusion complexes (cyclodextrin complexes) of inorganic oxy-acids can be obtained.